An airborne EM survey system generally includes a transmitter for generating a primary electromagnetic field that induces eddy currents in the earth. These eddy currents generate a secondary electromagnetic field or ground response. A receiver of the EM system then measures the response of the ground. The currents induced in the ground are a function of conductivity. By processing and interpreting the received signals, it is possible to study and estimate the distribution of conductivity in the subsurface.
EM measurements can be made in either frequency domain or time domain. In a frequency domain EM system, the transmitter generates a sinusoidal electromagnetic field at one or more frequencies. The amplitude and phase of the secondary field relative to the primary field are indicative of the subsurface conductivity. In a time domain EM system, transient pulses are applied to the transmitter during as ON-period to generate a primary electromagnetic field that induces a decaying secondary electromagnetic field. The receiver measures the amplitude and decay characteristics of the secondary field during an OFF-period of the pulses.
An airborne EM system's signal-to-noise ratio (SNR) is an important indication of the effective depth of exploration of the EM system and its ability to recognize and measure a potential target. Various systems and methods for improving SNR have been known in the art. For example, increasing the distance between the transmitter and receiver may reduce system noise thereby improving the SNR. In time domain systems, increasing the size of the transmitter loop may also help increasing the SNR. However, these conventional improvements are transmitter-dependent and usually result in increased size and complexity of the overall system.
Furthermore, the operation of the EM system in air may introduce other sources of noise. For example, erratic movement of the EM system receiver coil assembly in the earth's geomagnetic field may produce a signal in the receiver coils that cannot be distinguished from the induced fields (or earth response) the EM system is attempting to measure. This signal is then considered as noise and reduces the overall capability of the EM system to discriminate geological information.
International Patent Publication WO2009/135296A1 proposes a double suspension receiver coil apparatus wherein the receiver coil is elastically suspended within an inner tubular member, and the inner tubular member is elastically suspended within an outer tubular member. However, this proposed system is cumbersome to manufacture and assemble, and may not be as effective in reducing the torsional vibration, or rotational vibration, of the receiver coil frame.
Canadian Patent Application No. 2,650,183 proposes a receiver coil assembly comprising a receiver loop frame and two sets of ropes connecting the receiver loop frame to a hub member extending along a vertical axis of the receiver loop, wherein the two sets of ropes are axially spaced on the hub and are located on the opposite sides of the receiver loop. However, the proposed system is complex and may not be easily applied to receivers of large dimensions, or sealed up to accommodate various different receiver configurations, and as a result may not effectively isolate torsional and rotational vibrations from the receiver coil assembly.
Thus, there remains a need for an airborne EM system that provides increased signal-to-noise ratio and the depth of exploration, and a need for a simple, effective and versatile receiver coil assembly and system that reduce EM system noise by attenuating or eliminating unwanted motion or vibration in the receiver coil(s).